Abstract

Over the last 30 years window-to-wall ratio (WWR) has emerged as a foundational metric for both high performance buildings and minimum code standards. This study shows that WWR, when used without regard for the building depth or ceiling height, is a poor indicator of building energy consumption. This is especially true as energy codes become more aggressive. In addition, reliance on WWR guidelines often results in sub-optimal energy performance and badly daylighted spaces. We propose a new approach to determining optimal WWR by accounting for ceiling height and floor depth: Glazing-to-Volume Index (GVI). When considered in concert with façade orientation, climate, program, and envelope assemblies, GVI is shown to be a better tool for indicating both energy use and daylighting performance. GVI maintains the simplicity to be a useful early design phase rule-of-thumb as well as a building code prescriptive standard.

To find the optimal range of GVIs for various climates, we examined the relationship between Energy Use Intensity (EUI) and Spatial Daylight Autonomy (sDA) for an ASHRAE 90.1-2016 compliant office building. Parametric analyses were completed using EnergyPlus to simulate building energy use coupled with Radiance to simulate Daylight Autonomy. Lowest EUI and highest sDA GVI and WWR are reported for each analyzed climate. These results show that GVI is an improved indicator of performance over WWR. This is true especially for high performance buildings. GVI is also shown to be better suited both for design advice and code enforcement. In place of the current 40% WWR prescriptive standard, this paper proposes that model codes adopt climate- and orientation-specific GVI’s as prescriptive and performance standards.

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